Fringe Projection Methods (or Moiré Methods)
A number of non-contact optical measurement methods have been developed in the recent years and applied in many industrial and research domains. Some devices take for example advantage of the polarization states splitting technique for producing and shifting multiple sinusoidal Young's interference patterns that are projected on a scanned surface.
They are commonly applied to extract the range data of surfaces, from nanometers to kilometers scale. The projected-fringes techniques are among the most used approaches for measuring shape, surface profile and displacement of usual size objects. They allow robust, precise and fast whole-field acquisitions. Furthermore they benefit from the well-established procedures developed for interferometric systems such as phase-shifting and phase unwrapping algorithms.
In a typical way, one or multiple structured light patterns are projected on the surface to be analyzed. They are usually characterized by a periodic variation of the intensity in such a way that a specific phase can be associated to every enlightened points of the object. By recording the scene with a CCD or CMOS camera, it is possible to compare the phase distribution of the image points to the linearly growing phase of a non-distorted grid thanks to a first calibration step. This phase difference contains information required for a computation of the surface height variations based on triangulation formulae.
Favourable features for a good projection pattern are a perfect sinusoidal irradiance function, a very high contrast, high intensity irradiance and a large depth of field. The problem of contrast is especially critical when ambient light cannot be shut down, for instance in outdoor in-situ conditions.
In interferometric fringe projection systems, Young's interference pattern is a theoretically perfect sinusoid that can have a very high contrast. In addition, interferometric fringes are non-localized which means that the irradiance function and contrast remain unchanged whatever the projection distance so there is no depth of field problem. It makes an ideal base for a moiré-based technique device. The use of monochromatic laser light is also a beneficial approach for filtering relevant signal from ambient light.
However, dynamically shifting or scaling interferometric projection pattern often requires precise and complex electromechanical or optoelectronic systems whose repeatability and robustness is not assured. Internal vibrations are also a possible cause of trouble that compromises fringe stability. Simplicity, robustness, insensitivity to vibrations and low-cost are among the principal qualities of the required setup.
An overview of the art in structured light projection methods suitable to measure the 3D shape of objects (or 3D laser profilometry) is given in WO 2005/049840.
Shearography
On the other hand, the present invention also relates to the field of speckle-shearing interferometry or shearography, and is a valuable technique in the field of nondestructive testing. An overview of phase-stepping shearography methods is given in U.S. Pat. No. 6,717,681 B1.
A shearographic display produces the formation of an image made of two laterally-displaced images of the same object. Shearography is a full-field optical speckle interferometric procedure which is capable of measuring small deformations of a surface caused by stimuli such as vacuum or pressure, microwave, thermal, vibration, ultrasonic excitation and so on.
In a basic setup of an electronic shearography system, coherent laser light is spread out to uniformly illuminate a portion of the object's surface, reflects from the surface, passes through an optical shearing device and enters a CCD camera. Then one deforms the surface by one of the aforementioned mechanisms, such as heating for example. The surface slightly expands consequently and the effect of the deformation of the surface can be viewed under the form of an image on a video monitor or stored in the computer memory. This deformation of an object from one state to another one is in the micrometric range. Deformation of the surface can result from a subsurface flaw.
Research on Combining Both Shearography and Interferometric Fringe Projection
Shang et al (Beam-splitting cube for fringe-projection, holographic, and shearographic interferometry, Applied Optics, Vol. 40, No. 31 (2001), pp. 5615-5623) propose a beam-splitting cube for fringe-projection and shearographic interferometry. This proposed set up is very simple and needs a very good positioning of the optical element and only gives qualitative results.
A family of new non-contact optical measurement methods based on the polarization states splitting technique and monochromatic light projection as a way to overcome ambient lighting for in-situ measurement has been developed (Moreau et al, Interferometric fringes projection system for 3D profilometry and relief investigation, Proc. SPIE vol. 5857, pp. 62-69, 2005; WO 2005/049840). In this common path dynamic fringe projector, the key element is a separating polarization states prism coated on its hypotenuse with a Bragg grating. This set up has proven efficient and suitable for many applications as different as archeological survey and laboratory inspection. Despite these good results, this installation does not meet industrial needs such as robustness and fastness.
In order to get rid of these drawbacks, a new in-line interferometer which is still based on polarization states separation was built. A birefringent element, called Savart plate, allows to build a more flexible and robust interferometer [Michel et al, Nondestructive testing by digital shearography using a Savart plate, Photonics North, SPIE, 2009, Québec; Blain et al, Utilisation d'une lame de Savart pour un système de projection de franges interférométriques pour la mesure de forme 3D, CMOI 16-20 Nov. 2009, Reims, France; Renotte et al, Optical metrology devices based on an interferometer”, 3D Stereo Media déc. 2009, Liège; Blain et al, Using a Savart plate in optical metrology>>, Optical Engineering+Applications, SPIE, 1-5 Août 2010, San Diego, Calif. (Proceedings)].
The Savart plate has been selected as a new shearing device because it allows conservation of the above-mentioned advantages, i.e. in-line and almost common path setup, and the philosophy of the interferometer, i.e. shearing the object beam by separating two orthogonal linear polarization states. The Savart plate has been chosen among various usable birefringent elements because both sheared beams are propagating parallel to the optical axis of the device.
The shearing direction will be modified by rotating the Savart plate around the interferometer optical axis, and that without affecting the shearing amount, i.e. the sensitivity of the interferometer. The substitution of the above-mentioned coated prism by a Savart plate allows also improving the performances of the interferometer, because:                the polarization degree in transmission of birefringent elements is higher that the polarization degree of the coated prism;        the spectral range in the case of the Savart plate is wider (350-2500 nm) with respect to the coated prism spectral range (for example 532 nm);        the angular efficiency of the Savart plate is wider than the angular efficiency of the prism (better shearing at the edge of the field of view thanks to a numerical process);        the optical path difference between the beams sheared by a Savart plate is equal to zero for a (quasi)normal incidence. A shorter coherent length laser or possibly a good diode can be then employed by using a Savart plate as shearing device (spatially coherent light source).        
A Savart plate is made of two identical uniaxial birefringent crystals (quartz, calcite or any birefringent crystal) cut at 45° with respect to normal plane and are cemented in such a way that their optical axes are perpendicular. In this configuration, the ordinary ray of the first crystal becomes the extraordinary ray of the second one, and inversely. By birefringence, the incident object beam is sheared along a transversal direction with a shearing amount proportional to the crystals thickness (M. Born & E. Wolf, Principles of Optics, 6th ed. 1980, Pergamon Press, pp. 700-701).
In the steel industry there is an interest to develop a non-destructive testing (NDT) integrated control technology which would permit to detect depth defects, for example in polyurethane coating rolls used in continuous painting lines and also more generally for non-coated rolls. Prior art methods for detecting depth defects are unsatisfactory:                in the case of coated rolls, use of a transparent polyurethane layer for visual control is not always desired by the client;        ultrasound control time is lengthy and unacceptable for the industry (30-40 minutes per roll);        sound emitted by a roll after an external stimulus is empirical and unreliable.        